The invention concerns a doctor for dosing coatings of paper and cardboard webs, the body proper of which doctor, featuring the dosing surface, is fashioned as a bar. The bar has an irregular or cornered cross section, in section perpendicular to the longitudinal expanse of the bar, and with a sharp leaving or tearing edge. The dosing surface of the bar is flat or provided with a rounding so that its distance from the paper or cardboard web in running direction reduces continuously, except maximally for the area on its leaving edge.
Such a doctor is known from the U.S. Pat. No. 4,279,949. This doctor is provided with a bar-shaped doctor body which is pivotably mounted so as to enable an adjustment of the contact pressure of its sharp scraper edge on the leaving end of the doctor body. This doctor body may be provided with a wear-resistant surface, consisting specifically of carbide material. Other such doctors with an irregular cross section and bow-shaped doctor surface, in cross section perpendicular to the longitudinal expanse of the doctor bar, are known from EP-A 0109520 and DE-OS 36 20 374. Here, too, either the doctor surface or the entire doctor bar may consist of ceramic, specifically oxide ceramic material because it is especially wear-resistant.
The doctors known from the two formal publications were based on the concept that by means of a sharp, abrupt leaving edge on the end of the doctor surface of the doctor bar it would be possible to assure the uniformity of the coating application also at changing contact pressures. In all of the aforesaid publications the doctor surface was very smooth. But from driven, cylindrical doctor bars it is known to provide an exact dosing possibility for the doctor by means of circumferential grooving.
The problem underlying the invention is to make it possible in the initially mentioned prior devices to also achieve in doctor bars with irregular cross section a good dosing possibility at extremely uniform coating application, by a specific shaping of the grooves.